docs: def iter_sys(self):
          """
          Iterate over sys_name, overall_sys, histo_sys.
          overall_sys or histo_sys may be None for any given sys_name.
          """
          names = self.sys_names()
          for name in names:
              osys = self.GetOverallSys(name)
              hsys = self.GetHistoSys(name)
              yield name, osys, hsys